Conversion of high molecular weight petroleum feeds to more valuable products is important to petroleum processes such as fluidized catalytic cracking (FCC) and coking. In the FCC process, high molecular weight feeds are contacted with fluidized catalyst particles in the riser reactor of the FCC unit. The contacting between feed and catalyst is controlled according to the type of product desired. In catalytic cracking of the feed, reactor conditions such as temperature and contact time are controlled to maximize the products desired and minimize the formation of less desirable products such as light gases and coke.
Miscellaneous FCC riser and reactor designs have been utilized. However, with the advance of zeolitic cracking catalysts with greatly improved cracking activity, most modern FCC reactors utilize a short-contact time cracking configuration in which the amount of time that the catalyst and the FCC feedstream are in contact is limited in order to minimize the amount of excessive cracking which results in the increased production of less valued products such as light hydrocarbon gases as well as increased coking deposition on the cracking catalysts. Most short-contact time FCC configurations utilize a riser cracking configuration wherein the catalyst is contacted with the FCC reactor hydrocarbon feedstock in a riser and the catalyst and the hydrocarbon reaction products are separated shortly after the catalyst and hydrocarbon mixture leaves the riser and enters the reactor. Although there are many different FCC reactor designs in use, most use mechanical cyclones internal to the reactor to separate the catalyst from the hydrocarbon reactor products as quickly and efficiently as possible. This rapid separation process has the benefits of both minimizing post-riser reactions between the catalyst and the hydrocarbons as well as providing a physical means for separating the products to be sent for further processing from the spent catalyst which is sent to a regenerator stage prior to reintroducing as regenerated catalyst back into the reaction process.
This improved catalyst technology has led to the ability for existing FCC units to improve throughput in the reactor section of existing equipment. However, this improved reaction section performance has resulted in shifting process rate bottlenecks to other existing FCC equipment which may prevent the reactor section from operating at improved or maximum rates. Non-limiting examples of equipment that are the focus of rate improvement modifications are the FCC reactor stripping section and the FCC regenerator section.
The FCC reactor stripping section, in particular, is important to maximizing the throughput of the FCC reaction/regenerator section. The FCC stripper utilizes a stripping medium, usually steam, to strip hydrocarbons from the spent FCC catalyst prior to the catalyst being sent to the FCC regenerator. In the FCC regenerator, the spent catalyst is subjected to temperatures from about 1100 to about 1400° F. (593 to 760° C.) in order to regenerate the catalyst activity by burning the residual hydrocarbons and coke deposits from the catalyst prior to sending catalyst, in its regenerated state, back to the reaction stage of the FCC process. Whatever hydrocarbons are not effectively stripped off of the catalyst in the stripping section are sent to the regenerator zone resulting in an increased combustion load on the FCC regenerator as well as having several other adverse impacts to an FCC unit. If the particular FCC process is regenerator rate limited, insufficient stripping of hydrocarbons in the FCC stripper can be a direct cause of loss in overall unit throughput.
The efficiency of the stripping section of the process is therefore very important to the overall throughput of the FCC process as well as to the efficiency and environmental performance of an FCC unit. In addition to the rate limiting aspect of improper or inefficient hydrocarbon stripping mentioned above, inefficient FCC stripping can also result in loss of product, increased emissions, increased steam usage, and related detrimental affects. Any residual hydrocarbon product that is not removed from the spent catalyst in the FCC stripper becomes lost product. If the hydrocarbon residue is not stripped prior to leaving the stripper section, it is combusted in the FCC reactor section. Besides the corresponding loss of product, this additional combustion is undesired as it increases contaminant concentrations in the regenerator flue gas and/or increases the regenerator flue gas rate resulting in increased air pollutant emissions from the FCC unit. Additionally, an inefficiently designed FCC stripping section will result in the use of an excess amount of stream in the FCC stripper and reactor. This excess steam can result in a decrease overall hydrocarbon processing capacity in the associated FCC fractionator tower as well as increasing the amount of water that must be removed from the hydrocarbon product and subsequently treated prior to disposal or reuse.
There have been apparatus designs intended to improve the catalyst/stripping gas contact in the FCC stripper. Many “disc and donut” stripper tray designs have been proposed to improve the stripping process associated with the “annular riser” FCC reactors. An FCC annular riser reactor has the riser section entering through the bottom of the reactor and rising up through the center axis of the FCC reactor. Examples of annular tray designs can be seen in U.S. Pat. Nos. 5,531,884, and 6,248,298. In contrast with the riser configuration of the FCC axial riser design, an external riser FCC reactor configuration is designed where the main feed/catalyst riser or risers does not enter axially through the bottom of the FCC reactor, but instead is external to the FCC reactor until it enters the reactor, usually in the side of the reactor, somewhere in the upper section of the FCC reactor. An additional, somewhat hybrid design is what is termed herein and described more fully as an FCC “S” riser reactor design.
In addition to the various tray designs, packing designs have been proposed to increase the stripping efficiency of an FCC stripper. U.S. Pat. No. 5,716,585 and United States Publication Number US 2005/0205467 illustrate the use of packing configurations in an FCC stripper. While these designs may be theoretically efficient, packing in an FCC stripper service can have several significant disadvantages as compared to stripper tray or shed designs. The packing can be prone to plugging, resulting in capacity restrictions, or bypassing and channeling, which can result in poor long-term stripping efficiencies, especially when considering that an average FCC unit is designed to run from 3 to 6 years between reactor maintenance downtime cycles. In addition, grid packing can be expensive and difficult to install as compared to tray or shed designs, as well as being an obstruction to internal access of the reactor during maintenance cycles requiring removal, cleaning, and reassembly or replacement during periodic FCC reactor maintenance cycles.
Therefore, there exists in the industry a need for an improved stripper section design for an external riser FCC reactor that is simple to install and maintain, is not prone to plugging or channeling, and has improved hydrocarbon/catalyst separation efficiencies.